In industrial communication systems availability and reliability are key issues for time-critical applications such as vehicle control, drive control or substation automation because a failing communication system can lead to an interruption of the application, a shutdown of an industrial plant, or a crash of a vehicle controlled by the latter. Therefore, communication network redundancy is an important feature of industrial communication systems demanding high availability, in particular those systems using Ethernet-based communication with commercial switches.
A key factor of a redundant system is the recovery delay in case of failure, i.e. the time it takes until a redundant component takes over the role of a failed component. Time-critical processes have specified recovery delays on the order of milliseconds or, preferably, even a seamless recovery invisible to the plant. If a delay is too long the interruption in service can trigger a plant shutdown or otherwise cause damage.
In case of failure, seamless operation can be provided by using two local area networks operated in parallel. This involves doubling the electrical or optical transmission cables and switches, as well as at least the physical layer of the corresponding protocol stack of the devices, and attaching a device to each of the two local area networks through two independent transceivers and lines. For each frame to be transmitted, a sender sends two frames nearly simultaneously over the two independent lines while the receiver listens to both lines, accepts whichever frame of a pair of redundant frames comes first, and discards the later frame. Operation in case of failure of one network element is seamless, since there is no need to repeat a frame in case of disruption of one path. The tagging of redundant frames offers a complete supervision of both redundant lines with normal traffic, ensuring a high coverage. This method has been described in patent application WO 2006/053459 and standardized as Parallel Redundancy Protocol (PRP) in IEC standard 62439. However, PRP requires a complete and hence rather uneconomical duplication of the local area network.
In a ring network, switching nodes have two communication ports connecting to two neighbor nodes and being able to forward frames from one port to the other according to bridging rules. A switching element can also be integrated within the node where the source or destination application runs, thus forming a switching end node. With full-duplex links, the ring network can be operated in either or both directions. As a result, the ring network offers resiliency against link failure. Ring protocols are known from a Fiber Distributed Data Interface (FDDI) or Token Ring, and can also be implemented with Ethernet, in which case protocols such as Rapid Spanning Tree Protocol (RSTP) (IEEE 802.1D) ensure that frames cannot circulate indefinitely on the ring. A ring topology provides a cost-effective redundancy, since only one additional link is needed to protect against any single link failure.
WO 2004/056049 discloses a substation-specific ring network normally operated in a first communication direction along a so-called primary ring. In the event of a fault on the primary ring, a part of the latter may be short-circuited by a part of a secondary ring. This ring network offers the same availability as PRP against link failures. The switchover operation, however, introduces a long recovery delay. In general, any restoration based on the redirection of the information flow on the still available links after detection of a failure is likely to introduce a recovery delay that can be incompatible with the specifications of a higher level application.
Nodes belonging to the ring can be specified to include two network interfaces, a property not shared by commercial off-the-shelf non-ring devices such as printers, servers or laptop computers. FR 2893205 discloses to connect such a device in a point-to-point link to a dedicated switching-inserting node which in turn is connected to a first and a second unidirectional ring. The two rings jointly realize bidirectional data transmission with seamless redundancy. However, since each switching node introduces a forwarding delay, the number of non-ring devices in ring networks is generally limited.